Software & Stacks

Kamailio (SIP server / softswitch)

Kamailio (formerly OpenSER) is a high-performance SIP server. It is not a PBX — it has no media, no IVR, no voicemail. It is a SIP signaling proxy/router/registrar that handles tens of thousands of calls per second on commodity hardware, and it is the de-facto standard Class 4 softswitch in the open-source world.

What Kamailio does

SIP proxy / router: takes SIP messages, applies routing logic (in a C-like config language), forwards them.
Registrar: handles SIP REGISTER from millions of endpoints.
Load balancer / dispatcher: spreads INVITEs across upstream gateways with weight + probing.
Authentication: digest auth backed by DB or radius.
Anti-fraud / rate-limiting: pike, htable, ratelimit modules.
Topology hiding (B2BUA mode): with mod_b2b_logic.

What it does NOT do: media. It only sees SIP signaling. RTP flows directly between endpoints (unless you put rtpengine in line for media anchoring).

Where Kamailio fits

In front of Asterisk/FreeSWITCH cluster — Kamailio handles registration + routing, Asterisk handles features. The standard scaling pattern.
As a wholesale carrier softswitch — routing INVITEs between carriers based on rate decks.
As a SIP load balancer in front of a cluster.
As a SIP-WebSocket gateway for browser-based SIP.

Configuration model

Configuration is a single big file (kamailio.cfg) in a C-like syntax with route{} blocks. Logic is procedural: when an INVITE arrives, it goes through the main route block, can branch to sub-routes, can hit modules that mutate the message. Steeper learning curve than Asterisk's dialplan but vastly more flexible.